In a transport vehicle, for example, a dump truck used as a mining machine, a parking brake of a negative control type is provided in order to maintain a stop of a vehicle body on a slope way and the like. In the parking brake, a solenoid valve of a spool type is operated to actuate or release the brake with a hydraulic pressure using a brake caliper. Since the parking brake is the negative control type, the parking brake is actuated during a vehicle body stop and is always released during vehicle body traveling. In general, the mining machine operates continuously for a long time (about 8 hour to 12 hours). Therefore, the parking brake is not actuated during the operation.
Dust sometimes contaminates pressure oil. Therefore, when a spool does not operate for a long time, the dust collects between the spool and a valve main body that slidably covers the spool (a silting phenomenon). The silting phenomenon causes sticking of the spool to the valve main body and becomes a factor of prevention of the actuation of the parking brake. The dust is present in the pressure oil because objects in the air intrude into the pressure oil during work. In particular, the mining machine is often used in a poor environment. Therefore, dust tends to intrude into a hydraulic circuit during work.
A brake control device that prevents such sticking of the solenoid valve is described in Patent Document 1. However, the brake control device described in Patent Document 1 is based on an ABS (Anti-Lock Breaking System). As a situation in which the control of the brake control device works, traveling of the vehicle body at the speed of 40 Km/hour is assumed. However, in the mining machine, when the solenoid valve is demagnetized during traveling and the parking brake is actuated, this causes breakage of the brake itself. In Patent Document 1, suppression of noise is also taken into account. However, in the mining machine, noise is not a problem. Therefore, there is a demand for a technique for prevention of the silting phenomenon peculiar to the mining machine.